Mars

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Views from the Viking Orbiters: centered on the Schiaparelli Crater (left) and centered on Valles Marineris (right).

Mars is about half the diameter of the Earth and has 1/10th the Earth's mass. Mars' thin atmosphere (just 1/100th the Earth's) does not trap much heat at all even though it is 95% carbon dioxide (CO₂). The other 3% is nitrogen (N₂). Because the atmosphere is so thin, the greenhouse effect is insignificant and Mars has rapid cooling between night and day. When night comes the temperature can drop by over 100 K (180° F)! The large temperature differences create strong winds. The strong winds whip up dust and within a few weeks time, they can make dust storms that cover the entire planet for a few months. Two "before-after" image sets are shown below. The first pair is from the Mars Global Surveyor of the Tharsis bulge side of the planet. The "before-after" images are about 1.5 months apart. The second pair is from the Hubble Space Telescope (HST) of the other side of the planet. The "before-after" images are about 2.5 months apart and the truly global dust storm was still going on. You can see the dust storm beginning in the HST image in the left image in the Hellas Basin at about the 4 o'clock position.

Liquid Water

Color view of river drainage system from Viking orbiter on left and black-and-white zoom-in of Nanedi Vallis canyon using Viking (wide-field view) and Mars Global Surveyor (magnified view) images on right.

What makes Mars so intriguing is that there is evidence for running liquid water in its past. Some geologic features look very much like the river drainage systems on Earth and other features points to huge floods. The Mars Pathfinder studied martian rocks in the summer of 1997 and found some rocks are conglomerates (rocks made of pebbles cemented together in sand) that require flowing water to form. Abundant sand also points to widespread water long ago. More recently, the larger and more advanced Mars Exploration Rovers (one called "Spirit" and the other called "Opportunity") have further strengthened the conclusion that there was liquid water on Mars in the past. Highly magnified images of the rocks examined by Opportunity show a particular type of rippling patterns on the rocks that are formed under a gentle current of water instead of wind. Furthermore, detailed chemical analysis of the compositions of the rocks by Opportunity show that they formed in mineral-rich water when the water got very concentrated with the minerals as the water evaporated. Where there is liquid water, there is the possibility for life to arise.

Tiny structures in a meteorite that was blasted from Mars in a huge impact of an asteroid look like they were formed by ancient simple lifeforms. However, there is still a lot of debate among scientists on that but strong evidence of contamination by terrestrial organic molecules has probably killed the possibility of conclusive proof of martian life in the meteorite. The dissenters are not wanting to be party poopers. They just want greater certainty that the tiny structures could not be formed by ordinary geologic processes. The great importance of discovering life on another world warrants great skeptism---``extraordinary claims require extraordinary evidence''. The search for martian life will need to be done with a sample-return mission or experiments done right on Mars.

Mars' surface air pressure is much too low for liquid water to exist now. At very low pressure, water can exist as either frozen ice or as a gas but not in the intermediate liquid phase. If you have ever cooked food at high elevations using boiling water, you know that it takes longer because water boils at a lower temperature than at sea level. That is because the air pressure at high elevations is less. If you were several miles above the Earth's surface, you would find that water would boil (turn into steam) at even room temperature! The fact that Mars had liquid water in the past tells us that the early Martian atmosphere was thicker and the surface was warmer from the greenhouse effect a few billion years ago. Some of the topics for follow-up research are: how long was there liquid water present on the surface; when did the liquid water disappear from the surface; how widespread was the liquid water; how much liquid water was there; and were there repeated episodes of liquid water appearing and then disappearing.

Life may have started there so current explorations of Mars are focussing on finding signs of ancient, long-dead life. Any lifeforms living now would have to be living below the surface to prevent exposure from the harsh ultraviolet light of the Sun. Mars has no protective ozone layer, so all of the ultraviolet light reaching Mars can make it to the surface. The Viking landers that landed in 1976 conducted experiments looking for biological activity, past or present, in the soil but found the soil to be sterile and more chemically reactive than terrestrial soil from the action of the harsh ultraviolet light. What changed Mars into the cold desert of today?

Three views of Mars from three generations of Mars explorers: Viking 2 lander on top left, Mars Pathfinder on top right,
and Opportunity on bottom (much less rocky terrain---long picture!).

Mars Exploration Rover Sites movie (select the link to view a Flash movie showing where the Mars Explorations Rovers landed)

Atmosphere Escape

(1)

Mars' low gravity let the atmosphere leak away into space;

(2)

A lot of impacts of asteroids blasted part of the atmosphere away. Such large impacts occurred very frequently in the early solar system several billion years ago. The energy of the impacts could have been enough to push the gas away from a planet with small gravity.

(3)

Mars had a reverse greenhouse effect, called a runaway refrigerator, occur. Since Mars was slightly further from Sun than the Earth, Mars' initial temperature was lower. This meant that the water vapor condensed to form a liquid water layer on the surface. Gaseous carbon dioxide dissolves in liquid water and can then be chemically combined with rocks. This would have happened on Mars long ago. The removal of some of the carbon dioxide caused a temperature drop from the reduced greenhouse effect. This caused more water vapor to condense, leading to more removal of atmospheric carbon dioxide and more cooling, etc. The runaway feedback process is called a runaway refrigerator. This runaway process occurred a few billion years ago, so Mars has been cold for a long time. Mars' water is now frozen in a permafrost layer below the surface and the atmosphere is very thin.

The runaway refrigerator is described in a flowchart on the Earth-Venus-Mars page. The flowchart up to the last dashed arrow occurred several billion years ago. The box at the end describes the current state: frozen water and carbon dioxide below the surface and a very thin atmosphere.

(4)

The atmosphere was slowly eaten away by the solar wind that is able to directly reach the upper atmosphere because Mars does not have a magnetic field. The fast-moving solar wind particles hit the upper atmosphere particles with such force to kick them to speeds faster than the escape velocity.

(5)

A combination of these effects.

Human explorers will need to use spacesuits on Mars' surface. The low pressure would kill them in a fraction of a second without something to provide an inward pressure on their bodies (far faster than movies like Total Recall would have you believe, but moviemakers don't want to kill off the bad guy too quickly and Arnie must survive!). Explorers will also need to contend with temperatures that are way below the freezing point of water even during the day and have enough shielding to block the abundant ultraviolet light from the Sun.

And just for fun: Zoom in to the "Face on Mars". The "Face on Mars" is a remnant massif that attracted a lot of attention in the 1990s when a Viking 1 Orbiter picture of it, that made it appear like a face had been carved onto the martian surface, circulated around the web. Those with an over-active imagination thought the feature was artificial (made by ancient martian astronauts) and conspiracy theories were created of a NASA cover-up. The Viking image has poor resolution, poor lighting, and a number of "bit errors" that create black speckles, a couple of which were located at the feature's "nostril" and a "dimple" on its chin. Later much higher resolution images from the Mars Global Surveyor and Mars Express spacecrafts show it to be a naturally-occuring geologic formation. The "`Face on Mars' Zoom In" page shows where the "Face" is on Mars and increasingly higher-resolution views of the feature.

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last updated: June 1, 2007